A distinction is made between atomizing burners and gasification burners. In atomizing burners, the fuel is atomized with a nozzle and combusted in a combustion chamber into which air is supplied. Since the atomizing output of the nozzle can be varied only within narrow limits, atomizing burners have the disadvantage that their output cannot be continuously controlled. Nor can they be built for very small heat requirements. The smallest nozzles are dimensioned for an oil throughput of approximately 1.4 kilograms per hour. Because the output of atomizing burners cannot be varied continuously, atomizing burners are operated intermittently whenever the heat requirement is low. Since the intervals between periods of operation cannot be made arbitrarily brief, relatively large boilers are required as energy storage means. Intermittent operation has the disadvantage that switching the burner on and off repeatedly causes severe alternating temperature stresses on the materials, as well as a high burden of soot and toxic substances for the boiler, chimney and environment. Incomplete combustion and soot formation, which occur particularly in the startup phase, are highly detrimental to the overall efficiency of a heating system. Radiation losses in the large boilers contribute further to reducing overall efficiency.
In contrast to the atomizing burners described above, gasification burners as a rule have the advantage that they can be controlled continuously, down to very low outputs, in accordance with the heat requirement. In the combustion of gasified fuel, the emission of toxic substances, such as uncombusted hydrocarbons and soot, is also reduced considerably.
Despite the many advantages of gasification burners, they are used only to a limited extent. One major reason for this is that most gasification burners require a great deal of maintenance. In gasification burners, undesirable deposits generally tend to form in the gasification chamber, which soon impair gasification efficiency and hence burner operation considerably.
In U.S. Pat. No. 4,421,475, to which European Pat. No. 0 036 128 corresponds, a gasification burner having an electrically heatable gasification chamber is described. The temperature of this gasification chamber is measured by a temperature sensor and kept at an optimal value by means of a control device, to prevent fuel carbonization. A further provision for avoiding carbonization is that the gasification chamber has no air inlet openings. Furthermore, a rotatable cleaning device in the form of a wiper is housed in the gasification chamber. This wiper serves to distribute the fuel finely over the heated gasifier walls and prevent deposits from forming, so as to avoid the detrimental influence of deposits on fuel evaporation. The gas formed in the gasification chamber leaves the chamber at relatively high speed through a nozzle. The air required for combustion is provided by a fan. The burner described has the disadvantage of requiring a relatively large amount of electrical energy for evaporation of the fuel. Burners of this type are also relatively expensive, because they require a temperature sensor and a temperature controller. Compared with other gasification burners, where the mixing of fuel and air takes place prior to combustion in the combustion chamber, the combustion of the gas emerging from a nozzle at relatively high speed has the disadvantage of generating a relatively large amount of noise. Cold starting problems can also arise, because the air is not heated, or is heated only insignificantly, prior to the combustion. Another disadvantage is that upon shutoff, gasified fuel can continue to burn with a sooty flame. It is also possible for still-uncombusted hydrocarbons to emerge from the gasification chamber after the shutoff.
European Pat. No. 0 067 271, Noack discloses a continuously controllable oil burner with an electrically heated evaporation device having air inlets, which is monitored by a thermostat. This evaporation device is in the form of a beaker, with air inlets provided on the bottom of the beaker. A rotating cylinder for oil distribution is located in this beaker. This cylinder fills the entire evaporation chamber in the beaker except for a small gap. For oil distribution, oil is supplied to the rotating cylinder via a hollow drive shaft, and then ejected by centrifugal force from the radial bores in the rotating cylinder onto the inner walls of the evaporation chamber. Oil burners of this type have not attained commercial application, however. A disadvantage is that the gasification chamber tends to become soiled, which impairs the entry of air or the exit of the air and gas mixture. Since the pressure difference between the air inlet and the air and gas mixture outlet is very small, even slight soiling results in a sooty flame. Another disadvantage is that the rotating cylinder absorbs a large quantity of heat via the cylinder surface and transmits it via the drive shaft to the drive motor, which can be damaged thereby, unless expensive provisions for protecting it are made. The necessity for thermostat monitoring of the gasifier contributes further to increasing the initial cost of the burner.
U.S. Pat. No. 3,640,673 describes a burner for a kerosene stove in which a fan is located in the gasification chamber, which is heatable electrically and by the flame of the burner. A relatively large space exists between the periphery of the fan and the heated wall surface of the gasification chamber. An atomizer plate for the fuel is located on the drive shaft for the fan. When fuel is sprayed onto the atomizer plate during operation, the plate distributes the fuel into fine droplets, which are spun outward by centrifugal force. In this process they are mixed by the fan with the preheated air flowing into the gasification chamber. Since the distance between the periphery of the fan and the heated wall face of the gasification chamber is relatively large, most of the fuel droplets evaporate without ever coming into contact with the wall surface. The few fuel droplets that do strike the heated wall of the gasification chamber then evaporate there. It is disadvantageous that deposits form on the wall, which impair the evaporation, especially in the startup phase, when the gasification chamber is heated only electrically. This can then cause startup problems. Furthermore, uncombusted hydrocarbons occur both in the startup phase and in the shutoff phase. A further disadvantage of the described burner is that it can be operated only with Kerosene, is practically an atmospheric burner, and thus is unsuitable for use with a boiler.
European Patent Application No. 0 166 329 of Fullemann, which was published on Jan. 2, 1986, describes a gasification burner in which a rotor, provided with blades that extend to the vicinity of the heatable wall of the gasification chamber, is provided. The gasification chamber has an air inlet. The fuel supplied via the rotor shaft is finely distributed by the rotor and mixed with compressed air, evaporating in the hot gasification chamber. The mixture can escape at relatively high pressure through openings in a burner plate and burns with a low-noise, blue flame.
For the sake of completeness, the oil burner described in Swiss Pat. No. 628 724 should also be noted, which although it is an atomizing burner also shares some characteristics of a gasification burner. It has the intrinsic disadvantage of atomizing burners of not being controllable over a wide output range. Even in the lowest output range, it still requires a relatively high throughput of 1.6 to 2.1 kilograms of oil per hour.
For gasification of the atomized oil droplets, a mixing tube and a flame tube are provided coaxially with the nozzle. In operation, the oil is injected through the nozzle into the mixing tube, into which the air required for combustion is also blown. A flame then forms at the end of the mixing tube. A portion of the hot combustion gases is then recirculated to the beginning of the mixing tube and mixed there with the mixture of atomized oil and air for the sake of heat exchange. Because of the recirculation of a portion of the combustion gases, this burner enables extensive gasification of the oil droplets in the mixing tube and thus better combustion with less soot. However, this advantage is attained at the cost of an increased formation of nitrogen oxides (NO.sub.x). The burner in fact requires a long flame tube. Since expansion of the flame takes place only after it emerges from the flame tube, there is a relatively large flame zone at very high temperatures, which favors the formation of nitrogen oxides. As already mentioned, the burner also has the disadvantage of not being controllable over a wide output range. In the lowest output range, it requires a relatively high oil throughput of 1.6 liters per hour. This burner has additional problems in startup and shutoff, a factor that is all the more serious since the burner has to be operated intermittently. One problem in startup is the ignition of the oil droplets flowing out of the atomizer nozzle. Unlike a conventional atomizing burner, optimal disposition of the ignition electrodes is prevented in this case by a wall having an air aperture plate. Hence there is a great danger that ignition will not occur even in repeated starting attempts. A further problem is that at startup, the mixing tube is cold and thus has no evaporation capacity. The flame is therefore extremely sooty until the mixing tube has attained a high temperature and is capable of evaporating the oil that strikes it. When the burner is shutoff, the oil dripping from the nozzle continues to burn with an extremely sooty flame. Since at shutoff the mixing tube located near the nozzle is still red-hot, a great deal of heat radiates from toward the nozzle, which can lead to carbonization of fuel in the nozzle. This can clog the nozzle, especially when it is small.
German Patent Disclosure 3 346 431 discloses a burner having a rotating evaporator cup. This cup is closed on the flame side and has an outlet for the evaporated fuel only on the motor side. The evaporator cup is surrounded by an annular deflection chamber for the air supply. Gasified fuel and air then flow between the evaporator cup and the flame tube in two concentric flows of annular cross section, strike a baffle ring, mix, and then form a flame. The disadvantage is that the evaporator chamber is not subjected to a forceful flow of hot gases, and so deposits form there that soon impair the function of the burner. In particular, a major emission of uncombusted hydrocarbons occurs upon shutoff of the burner
French Pat. No. 2 269 029 also discloses a burner having a rotating evaporator cup that is closed on the flame side. The evaporator cup is lined on the inside with a wire mesh, which serves to prevent an outflow of the fuel. This burner needs a strong blower that requires a relatively large amount of energy, because the fresh air and the air and gas mixture are deflected several times. Another disadvantage is that after shutoff of the burner, a large amount of fuel is still evaporating from the wire mesh, which was previously swept with air and therefore has remained relatively cool; once again, a major emission of hydrocarbons is the result.
U.S. Pat. No. 2,535,316 discloses a burner having a spherical gasification chamber, which rotates slowly. The fuel flowing through a line forms an oil bath at the bottom of the chamber, from which the lighter fractions evaporate. The remaining tar and coke residue forms a thin layer on the chamber wall, and with the slow rotation of the chamber, this layer migrates slowly upward. There, a flow of air meets this layer and burns it off continuously. The disadvantage here is that when the burner is shut off the oil bath causes a major emission of soot, tar and uncombusted hydrocarbons.